1. Field of the Invention
The present invention relates to an energy-trap piezoelectric resonator used as, for example, an oscillator or a band-pass filter, and more particularly, to an energy-trap piezoelectric resonator in which resonance electrodes disposed opposite to each other with a piezoelectric substrate located therebetween have improved shapes.
2. Description of the Related Art
Conventionally, an energy-trap piezoelectric resonator suitable for use in an oscillator operating in a MHz band uses a non-fundamental harmonic of a thickness extensional vibration mode. For example, Japanese Patent Application Laid-Open (kokai) No. 4-216208 discloses an energy-trap piezoelectric resonator as shown in FIG. 16.
An energy-trap piezoelectric resonator 51 shown in FIG. 16 uses a piezoelectric substrate 52 polarized in the thickness direction thereof. A resonance electrode 53 is disposed on the upper surface of the piezoelectric substrate 52 at the center thereof, while a resonance electrode 54 is disposed on the lower surface of the piezoelectric substrate 52 at the center thereof. The resonance electrodes 53 and 54 have circular shapes, are the same size, and are positioned opposite to each other with the piezoelectric substrate 52 disposed therebetween.
In the energy-trap piezoelectric resonator 51, the portion of the piezoelectric substrate 52 where the resonance electrodes 53 and 54 overlap each other constitutes a vibration portion. Upon application of an AC voltage to the resonance electrodes 53 and 54, thickness extensional vibration is generated in the resonator 51. Further, partial electrodes 55 are provided in order to utilize a non-fundamental harmonic of the thickness extensional vibration while lowering the response to the fundamental wave of the thickness extensional vibration. The partial electrodes 55 are disposed on the upper and lower surfaces of the piezoelectric substrate 52 so as to extend along the center portions of the corresponding longitudinal edges. Due to the mechanical load and the piezoelectric short-circuit effect created by the partial electrodes 55, vibration energy is attenuated, resulting in a decrease in the response to the fundamental wave of the thickness extensional vibration.
Meanwhile, Japanese Utility-Model Application Laid-Open (kokai) No. 5-25823 discloses a ceramic resonator as shown in FIG. 17. A ceramic resonator 61 of FIG. 17 utilizes the fundamental harmonic of thickness extensional vibration and has a structure in which circular resonance electrodes 63 and 64 having the same dimension are respectively disposed on the opposite main surfaces of a rectangular piezoelectric substrate 62. In the ceramic resonator, a circular resin layer 65 is disposed on the upper resonance electrode 63 such that the circular resin layer 65 has a diameter equal to or smaller than that of the resonance electrode 63.
The resin layer 65 is provided to produce a damping effect by means of its mass load, thereby suppressing ripples within a band to be used.
As described above, in the conventional energy-trap piezoelectric resonators, the resonance electrodes disposed on the opposite main surfaces of the piezoelectric substrate are typically of identical shape and size, and are positioned so that the resonance electrodes are opposite to and overlap each other in their entirety with the piezoelectric substrate disposed therebetween.
In energy-trap piezoelectric resonators of the above-described types, since vibrations other than vibration to be used are considered spurious vibrations, suppression of such spurious vibrations is strongly demanded.
In the prior art technique disclosed in Japanese Patent Application Laid-Open No. 4-216208, a non-fundamental harmonic of thickness extensional vibration is used, and the above-described partial electrodes 55 are used in order to suppress the fundamental harmonic of the thickness extensional vibration which is an undesired, spurious vibration. However, when the size of the piezoelectric resonator 51 is reduced, there is insufficient space for providing the partial electrodes 55. That is, use of the partial electrodes 55 makes miniaturization of the piezoelectric resonator difficult.
Further, in the energy-trap piezoelectric resonator disclosed in Japanese Utility-Model Application Laid-Open No. 5-25823, undesired vibrations are damped through attachment of a resin layer 65. However, when the resin layer 65 is applied, the vibration to be used is damped as well. Although not disclosed in Japanese Utility-Model Application Laid-Open No. 5-25823, when a resonator that utilizes a non-fundamental harmonic of thickness extensional vibration is constructed through use of the piezoelectric resonator 61, the harmonic of thickness extensional vibration is damped, causing it to be impossible to achieve good resonance characteristics.
In addition, in the case of the piezoelectric resonator 61, since the resin layer 65 must be applied in order to suppress undesired, spurious vibrations, the manufacturing process becomes overly complex.